Behavioral observations and stable isotopes reveal high individual variation and little seasonal variation in sea otter diets in Southeast Alaska

Nl Laroche,Hc Pearson,Gl Eckert,Mc Rogers,Sl King

doi:10.3354/meps13871

Nl Laroche, Hc Pearson + Show 3 more

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https://doi.org/10.3354/meps13871

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Abstract

Two complementary approaches were used to assess year-round variation in the diet of sea otters Enhydra lutris around Prince of Wales Island (POW) in southern Southeast Alaska, a region characterized by mixed-bottom habitat. We observed sea otters foraging to determine diet composition during the spring and summer. Then, we obtained sea otter vibrissae, which record temporal foraging patterns as they grow, from subsistence hunters to identify year-round changes in sea otter diets via stable isotope analysis of carbon (δ13C) and nitrogen (δ15N). We compared the stable isotopes from sea otter vibrissae and sea otter prey items that were collected during spring, summer, and winter. Overall, year-round sea otter diet estimates from stable isotope signatures and visual observations from spring and summer were dominated by clams in terms of biomass, with butter clams Saxidomus gigantea the most common clam species seen during visual observations. Our results indicate that these sea otters, when considered together at a regional level around POW, do not exhibit shifts in the main prey source by season or location. However, sea otter diets identified by stable isotopes had a strong individual-level variation. Behavioral variation among individual sea otters may be a primary driving factor in diet composition. This study provides quantitative diet composition data for modeling predictions of invertebrate population estimates that may aid in the future management of shellfisheries and subsistence hunting and the development of co-management strategies for this protected species.

Highlights

The importance of sea otters Enhydra lutris in shaping nearshore marine ecosystems is well documented along the northeast Pacific coast (Estes & Palmisano 1974, Hughes et al 2013, Rechsteiner et al 2019, Hale et al 2019)
Analysis of stable isotopes from sea otter vibrissae in Southeast Alaska gave insight into year-round diets of sea otters around Prince of Wales Island (POW), information challenging to obtain from visual observations
Our study revealed no ecologically significant shifts in δ13C and δ15N according to season or location, indicating that sea otters maintained a relatively constant diet overall

Summary

Introduction

The importance of sea otters Enhydra lutris in shaping nearshore marine ecosystems is well documented along the northeast Pacific coast (Estes & Palmisano 1974, Hughes et al 2013, Rechsteiner et al 2019, Hale et al 2019). Sea otters are keystone predators with voracious appetites capable of causing major ecological shifts in nearshore marine ecosystems (Estes et al 2016). Sea otter recolonization patterns impact nearshore ecosystems (Estes & Duggins 1995). Sea otters once inhabited nearshore Pacific Ocean ecosystems from Japan to Baja California. In 1911, sea otter hunting was prohibited by the International Fur Seal Treaty (Burris & McKnight 1973). About 400 sea otters were relocated to 6 locations in Southeast Alaska in the 1960s (Burris & McKnight 1973, Jameson et al 1982). Aerial counts estimated that approximately 25 000 sea otters were present in Southeast Alaska in 2011, and very few locations within Southeast Alaska were estimated to be at carrying capacity at the time of that survey (Tinker et al 2019)

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